New benchtest: pthread locks

Performance benchmarks for various posix locks: mutex, rwlock,
spinlock, condvar, and semaphore.  Each test is performed with
an empty loop body or with a computationally "interesting" (i.e.
difficult to optimize away, and used just to allow lock code to
be "hidden" in the filler's CPU cycles).

benchtests/Makefile

@@ -31,7 +31,7 @@ ifneq (,$(filter yes,$(float128-fcts) $(float128-alias-fcts)))
 bench-math += expf128 powf128 sinf128
 endif
 
-bench-pthread := pthread_once thread_create
+bench-pthread := pthread_once thread_create pthread-locks
 
 bench-string := ffs ffsll
 
@@ -109,6 +109,7 @@ $(addprefix $(objpfx)bench-,$(bench-math)): $(libm)
 $(addprefix $(objpfx)bench-,$(math-benchset)): $(libm)
 $(addprefix $(objpfx)bench-,$(bench-pthread)): $(shared-thread-library)
 $(addprefix $(objpfx)bench-,$(bench-malloc)): $(shared-thread-library)
+$(addprefix $(objpfx)bench-,pthread-locks): $(libm)
 
 
 

benchtests/bench-pthread-locks.c

@@ -0,0 +1,554 @@
+/* Measure various lock acquisition times for empty critical sections.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#define TEST_MAIN
+#define TEST_NAME "pthread-locks"
+
+#include <stdio.h>
+#include <string.h>
+#include <limits.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <stdatomic.h>
+#include <sys/time.h>
+#include <math.h>
+#include "bench-timing.h"
+#include "json-lib.h"
+
+/* The point of this benchmark is to measure the overhead of an empty
+   critical section or a small critical section.  This is never going
+   to be indicative of real application performance.  Instead we are
+   trying to benchmark the effects of the compiler and the runtime
+   coupled with a particular set of hardware atomic operations.
+   The numbers from this benchmark should be taken with a massive gain
+   of salt and viewed through the eyes of expert reviewers.  */
+
+static pthread_mutex_t m;
+static pthread_rwlock_t rw;
+static pthread_cond_t cv;
+static pthread_cond_t consumer_c, producer_c;
+static int cv_done;
+static pthread_spinlock_t sp;
+static sem_t sem;
+
+typedef timing_t (*test_t)(long, int);
+
+#define START_ITERS 1000
+
+#define FILLER_GOES_HERE \
+  if (filler) \
+    do_filler ();
+
+/* Everyone loves a good fibonacci series.  This isn't quite one of
+   them because we need larger values in fewer steps, in a way that
+   won't be optimized away.  We're looking to approximately double the
+   total time each test iteration takes, so as to not swamp the useful
+   timings.  */
+
+#pragma GCC push_options
+#pragma GCC optimize(1)
+
+static int __attribute__((noinline))
+fibonacci (int i)
+{
+  asm("");
+  if (i > 2)
+    return fibonacci (i-1) + fibonacci (i-2);
+  return 10+i;
+}
+
+static void
+do_filler (void)
+{
+  static char buf1[512], buf2[512];
+  int f = fibonacci (5);
+  memcpy (buf1, buf2, f);
+}
+
+#pragma GCC pop_options
+
+static timing_t
+test_mutex (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_mutex_init (&m, NULL);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_mutex_lock (&m);
+      FILLER_GOES_HERE;
+      pthread_mutex_unlock (&m);
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static timing_t
+test_mutex_trylock (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_mutex_init (&m, NULL);
+  pthread_mutex_lock (&m);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_mutex_trylock (&m);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  pthread_mutex_unlock (&m);
+  return cur;
+}
+
+static timing_t
+test_rwlock_read (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_rwlock_init (&rw, NULL);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_rwlock_rdlock (&rw);
+      FILLER_GOES_HERE;
+      pthread_rwlock_unlock (&rw);
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static timing_t
+test_rwlock_tryread (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_rwlock_init (&rw, NULL);
+  pthread_rwlock_wrlock (&rw);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_rwlock_tryrdlock (&rw);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  pthread_rwlock_unlock (&rw);
+  return cur;
+}
+
+static timing_t
+test_rwlock_write (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_rwlock_init (&rw, NULL);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_rwlock_wrlock (&rw);
+      FILLER_GOES_HERE;
+      pthread_rwlock_unlock (&rw);
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static timing_t
+test_rwlock_trywrite (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_rwlock_init (&rw, NULL);
+  pthread_rwlock_rdlock (&rw);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_rwlock_trywrlock (&rw);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  pthread_rwlock_unlock (&rw);
+  return cur;
+}
+
+static timing_t
+test_spin_lock (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_spin_init (&sp, PTHREAD_PROCESS_PRIVATE);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_spin_lock (&sp);
+      FILLER_GOES_HERE;
+      pthread_spin_unlock (&sp);
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static timing_t
+test_spin_trylock (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  pthread_spin_init (&sp, PTHREAD_PROCESS_PRIVATE);
+  pthread_spin_lock (&sp);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_spin_trylock (&sp);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  pthread_spin_unlock (&sp);
+  return cur;
+}
+
+static timing_t
+test_sem_wait (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  sem_init (&sem, 0, 1);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      sem_post (&sem);
+      FILLER_GOES_HERE;
+      sem_wait (&sem);
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static timing_t
+test_sem_trywait (long iters, int filler)
+{
+  timing_t start, stop, cur;
+
+  sem_init (&sem, 0, 0);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      sem_trywait (&sem);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  return cur;
+}
+
+static void *
+test_condvar_helper (void *v)
+{
+  /* This is wasteful, but the alternative is to add the overhead of a
+     mutex lock/unlock to the overall iteration (both threads) and we
+     don't want that.  Ideally, this thread would run on an
+     independent processing core anyway.  The ONLY goal here is to
+     minimize the time the other thread spends waiting for us.  */
+  while (__atomic_load_n (&cv_done, __ATOMIC_RELAXED) == 0)
+    pthread_cond_signal (&cv);
+
+  return NULL;
+}
+
+static timing_t
+test_condvar (long iters, int filler)
+{
+  timing_t start, stop, cur;
+  pthread_t helper_id;
+
+  pthread_mutex_init (&m, NULL);
+  pthread_cond_init (&cv, NULL);
+  pthread_mutex_lock (&m);
+
+  __atomic_store_n (&cv_done, 0, __ATOMIC_RELAXED);
+  pthread_create (&helper_id, NULL, test_condvar_helper, &iters);
+
+  TIMING_NOW (start);
+  for (long j = iters; j >= 0; --j)
+    {
+      pthread_cond_wait (&cv, &m);
+      FILLER_GOES_HERE;
+    }
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+  pthread_mutex_unlock (&m);
+  __atomic_store_n (&cv_done, 1, __ATOMIC_RELAXED);
+
+  pthread_join (helper_id, NULL);
+  return cur;
+}
+
+/* How many items are "queued" in our pretend queue.  */
+static int queued = 0;
+
+typedef struct Producer_Params {
+  long iters;
+  int filler;
+} Producer_Params;
+
+/* We only benchmark the consumer thread, but both threads are doing
+   essentially the same thing, and never run in parallel due to the
+   locks.  Thus, even if they run on separate processing cores, we
+   count the time for both threads.  */
+static void *
+test_producer_thread (void *v)
+{
+  Producer_Params *p = (Producer_Params *) v;
+  long iters = p->iters;
+  int filler = p->filler;
+  long j;
+
+  for (j = iters; j >= 0; --j)
+    {
+      /* Aquire lock on the queue.  */
+      pthread_mutex_lock (&m);
+      /* if something's already there, wait.  */
+      while (queued > 0)
+	pthread_cond_wait (&consumer_c, &m);
+
+      /* Put something on the queue */
+      FILLER_GOES_HERE;
+      ++ queued;
+      pthread_cond_signal (&producer_c);
+
+      /* Give the other thread a chance to run.  */
+      pthread_mutex_unlock (&m);
+    }
+
+  return NULL;
+}
+
+static timing_t
+test_consumer_producer (long iters, int filler)
+{
+  timing_t start, stop, cur;
+  pthread_t helper_id;
+  Producer_Params p;
+
+  p.iters = iters;
+  p.filler = filler;
+
+  pthread_mutex_init (&m, NULL);
+  pthread_cond_init (&cv, NULL);
+
+  pthread_create (&helper_id, NULL, test_producer_thread, &p);
+
+  TIMING_NOW (start);
+
+  for (long j = iters; j >= 0; --j)
+    {
+      /* Aquire lock on the queue.  */
+      pthread_mutex_lock (&m);
+      /* Wait for something to be on the queue.  */
+      while (queued == 0)
+	pthread_cond_wait (&producer_c, &m);
+
+      /* Take if off. */
+      FILLER_GOES_HERE;
+      -- queued;
+      pthread_cond_signal (&consumer_c);
+
+      /* Give the other thread a chance to run.  */
+      pthread_mutex_unlock (&m);
+    }
+
+  TIMING_NOW (stop);
+  TIMING_DIFF (cur, start, stop);
+
+
+  pthread_join (helper_id, NULL);
+  return cur;
+}
+
+/* Number of runs we use for computing mean and standard deviation.
+   We actually do two additional runs and discard the outliers.  */
+#define RUN_COUNT 10
+
+static int
+do_bench_2 (const char *name, test_t func, int filler, json_ctx_t *js)
+{
+  timing_t cur;
+  struct timeval ts, te;
+  double tsd, ted, td;
+  long iters, iters_limit;
+  timing_t curs[RUN_COUNT + 2];
+  int i, j;
+  double mean, stdev;
+
+  iters = START_ITERS;
+  iters_limit = LONG_MAX / 100;
+
+  while (1) {
+    gettimeofday (&ts, NULL);
+    cur = func(iters, filler);
+    gettimeofday (&te, NULL);
+
+    /* We want a test to take at least 0.01 seconds, and try
+       increasingly larger iteration counts until it does.  This
+       allows for approximately constant-time tests regardless of
+       hardware speed, without the overhead of checking the time
+       inside the test loop itself.  We stop at a million iterations
+       as that should be precise enough.  Once we determine a suitable
+       iteration count, we run the test multiple times to calculate
+       mean and standard deviation.  */
+
+    /* Note that this also primes the CPU cache and triggers faster
+       MHz, we hope.  */
+    tsd = ts.tv_sec + ts.tv_usec / 1000000.0;
+    ted = te.tv_sec + te.tv_usec / 1000000.0;
+    td = ted - tsd;
+    if (td >= 0.01
+	|| iters >= iters_limit
+	|| iters >= 1000000)
+      break;
+
+    iters *= 10;
+  }
+
+  curs[0] = cur;
+  for (i = 1; i < RUN_COUNT + 2; i ++)
+    curs[i] = func(iters, filler);
+
+  /* We sort the results so we can discard the fastest and slowest
+     times as outliers.  In theory we should keep the fastest time,
+     but IMHO this is more fair.  A simple bubble sort suffices.  */
+
+  for (i = 0; i < RUN_COUNT + 1; i ++)
+    for (j = i + 1; j < RUN_COUNT + 2; j ++)
+      if (curs[i] > curs[j])
+	{
+	  timing_t temp = curs[i];
+	  curs[i] = curs[j];
+	  curs[j] = temp;
+	}
+
+  /* Now calculate mean and standard deviation, skipping the outliers.  */
+  mean = 0.0;
+  for (i = 1; i<RUN_COUNT + 1; i ++)
+    mean += (double) curs[i] / (double) iters;
+  mean /= RUN_COUNT;
+
+  stdev = 0.0;
+  for (i = 1; i < RUN_COUNT + 1; i ++)
+    {
+      double s = (double) curs[i] / (double) iters - mean;
+      stdev += s * s;
+    }
+  stdev = sqrt (stdev / (RUN_COUNT - 1));
+
+  json_attr_object_begin (js, filler ? "filler" : "empty");
+
+  json_attr_double (js, "duration", (double) cur);
+  json_attr_double (js, "iterations", (double) iters);
+  json_attr_double (js, "wall_sec", (double) td);
+  json_attr_double (js, "mean", mean);
+  json_attr_double (js, "stdev", stdev);
+  json_attr_double (js, "min_outlier", (double) curs[0] / (double) iters);
+  json_attr_double (js, "min", (double) curs[1] / (double) iters);
+  json_attr_double (js, "max", (double) curs[RUN_COUNT] / (double) iters);
+  json_attr_double (js, "max_outlier", (double) curs[RUN_COUNT + 1] / (double) iters);
+
+  json_attr_object_end (js);
+
+  return 0;
+}
+
+static int
+do_bench_1 (const char *name, test_t func, json_ctx_t *js)
+{
+  int rv = 0;
+
+  json_attr_object_begin (js, name);
+
+  rv += do_bench_2 (name, func, 0, js);
+  rv += do_bench_2 (name, func, 1, js);
+
+  json_attr_object_end (js);
+
+  return rv;
+}
+
+int
+do_bench (void)
+{
+  int rv = 0;
+  json_ctx_t json_ctx;
+
+  json_init (&json_ctx, 2, stdout);
+  json_attr_object_begin (&json_ctx, "pthread_locks");
+
+#define BENCH(n) rv += do_bench_1 (#n, test_##n, &json_ctx)
+
+  BENCH (mutex);
+  BENCH (mutex_trylock);
+  BENCH (rwlock_read);
+  BENCH (rwlock_tryread);
+  BENCH (rwlock_write);
+  BENCH (rwlock_trywrite);
+  BENCH (spin_lock);
+  BENCH (spin_trylock);
+  BENCH (sem_wait);
+  BENCH (sem_trywait);
+  BENCH (condvar);
+  BENCH (consumer_producer);
+
+  json_attr_object_end (&json_ctx);
+
+  return rv;
+}
+	    
+
+#define TEST_FUNCTION do_bench ()
+
+#include "../test-skeleton.c"